I just ran into a steam balloon. If helium is expensive and hydrogen against regulations, how about that? Add some steampunk trim to the cabin and off you go for the perfect retro-geek ride to space .

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

Actually, vacuum doesn't seem to be all that of an unrealistic proposition, especially if you're starting an finishing the flight where the atmospheric pressure is 1-2kPa. It would have to be able to withstand a pressure of say 5kPa, to give abundant margin, but... you're looking at potential significant mass and cost savings. Perhaps a grid of lightweight inflated struts within the body of the airship could reduce the required mass...

Interesting that you should mention this. A few years ago we worked on producing such a craft that would be floated aloft with helium in a latex balloon and then vent the helium in the balloon at 100,000'to inflate our struts supporting a 'vacuum craft' five times the volumn of the latex balloon. The project was sidelined as other projects required our attention.

Actually, I think you can do that without moving parts even. Make let's say six plastic-film closed cylinders of length l. Put them side by side lengthwise, and tie them together at the top and the bottom. Put them inside a spherical plastic-film ball of diameter D = l/pi, and attach them to the sphere along the 0, 60, 120, 180, 240 and 300 degree meridians. Add a line of length D between the North and South poles. Size everything so that the six cylinders, when inflated with helium to P < D/l bar, provide enough buoyancy to get off the ground. Put that amount of helium into the cylinders, pump the air out of the ball, and launch.

Now as the balloon rises, outside air pressure drops, and the cylinders expand, getting progressively longer. The surrounding envelope keeps them packed tightly together. At a certain altitude, they'll have expanded to a length D. Held together by the line between the North and South poles of the sphere, they cannot expand any further vertically, so they'll get pushed outward. If the outside pressure is low enough, the ball will expand, increasing volume and therefore buoyancy, so the balloon rises further, the cylinders get stiffer due to increased pressure differential and simultaneously the outside air pressure drops, so the ball expands further, and we keep rising until one of the cylinders bursts or the ball has reached full size and its buoyancy exactly compensates the weight. Optionally some of the helium can be vented along the way using an overpressure valve.

Further research: How big does this need to be to support a given payload? What's the best cylinder size? Is a sphere with cylinders along the meridians the most efficient shape, or would something else (such as a regular polyhedron) be better? How high can this thing go for a given size on the ground?

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus